The present invention relates to method for the preparation of a rare earth-cobalt based permanent magnet or, more particularly, to an improved method for the preparation of a rare earth-cobalt based permanent magnet with a relatively low content of copper as an additive element yet having a high coercive force and good squareness of the hysteresis loop.
As is well known, conventional rare earth-cobalt based permanent magnets are usually prepared with admixture of additional alloying elements of copper and iron in amounts of, for example, from 10 to 20% by weight and 5% by weight or larger, respectively, in consideration of the improvements obtained by the addition of copper to increase the coercive force and of iron to increase the residual magnetization of the magnets.
From the standpoint of the compatibility of these two magnetic parameters of a magnet, the addition of copper in a large amount is rather disadvantageous for the increase of the residual magnetization so that it would be very advantageous for rare earth-cobalt based permanent magnets if a high coercive force could be obtained even by the addition of a relatively small amount of copper, for example, less than 10% by weight.
It has been, however, a generally accepted conclusion that a sufficiently high coercive force can hardly be obtained with a rare earth-cobalt based permanent magnet of the alloying composition in which the content of copper as an additive element is low enough not to exceed 10% by weight or, in particular, not to exceed 8% by weight. There have been, of course, several attempts to overcome this difficulty. For example, certain improvement can be obtained in the coercive force of the magnet by a very much prolonged heat treatment or aging. Alternatively, the coercive force of a rare earth-cobalt based permanent magnet can be improved to some extent by the addition of certain alloying elements including zirconium, titanium, manganese, hafnium, tantalum and the like to the alloy composition for the powder metallurgical preparation of the magnet followed by heat treatment involving multi-step aging or continuous cooling-down. The highest coercive force of the magnets obtained by these improved methods unfortunately cannot exceed 7 kOe with an unavoidable problem of poor squareness of the magnetic hysteresis loop as one of the important characteristics of permanent magnets so that the permanent magnet prepared by these methods is not satisfactory in respect of the general performance of the magnets.